The present invention relates to a data transmission system having a plurality of parallel channels. More particularly, the invention is concerned with a selective fading protection system for maintaining stable operation of an automatic equalizer provided in a receiving system.
Parallel channel data transmission systems are widely used because of their high efficiency of data transmission and high selective fading characteristics. Since, in the usual case, a reduction of power due to fading takes place in only one or two channels, correction data from a preselected channel reserved for error correction can be used in place of the data from the power-failed channel thereby easily correcting the data. However, the automatic equalizer provided in the powerfailed channel operates out of control because it does not receive an input signal of sufficient strength to be effective for stable operation thereof, due to extreme deterioration of the S/N of the input signal. In consequence, the automatic equalizer cannot recover to a desired stable state even when the effective signal is received again after a relaxation of the selective fading, so that the equalizer remains in the unstable state or becomes stabilized at a different stable point, making it impossible to correct received data.